The invention relates to the technique of injection molding of shaped plastic parts, in particular sheet-like and possibly transparent shaped parts from plastic.
It is already known to use what is known as the film gate process to avoid sprue marks on sheet-like shaped plastic parts. In particular in the case of optical shaped parts, this process is customary, since sprue marks cannot be tolerated on optical surfaces. In the case of the film gate technique, the injection molding of the shaped part takes place laterally by means, of a film plate. The film plate has a runner, which opens into the component cavity at the edge. After the plastics compound has cooled down and the shaped part has been removed from the injection mold, it is provided with what is known as the film sprue, of a form corresponding to the runner in the film plate. After removal, the film sprue must be detached from the shaped part. On account of the lateral or edge-side injection molding performed in the case of the film gate technique, no sprue marks occur on the main surfaces of the shaped part. However, the additional working step that is required for detaching the film sprue, the loss of material caused by the film sprue, soiling of the shaped part that may occur when the film sprue is detached and optical defects at the points of detachment of the film sprue may be disadvantageous.
Another known sprue-free production process is that known as the fused deposition modeling process. In the case of this process, the “correct” amount of plastics material in the plastic state is deposited in one of the mold plates and distributed and compressed by applying the other mold plate. This process does not allow the production of high-quality shaped parts.
It is already known for the production of sheet-like shaped plastic parts to use what are known as compression processes. In compression processes, the liquid plastics compound is injected into a component cavity between the mold plates that is pre-enlarged or becomes larger during the injection operation, and is subsequently compressed in the direction of its thickness while a mold plate movement is performed. The injection of the plastics compound into the enlarged component cavity makes it easier for the cavity to be filled. In addition, the mold plate movement during the cooling phase compensates for shrinkage and so makes it possible to produce shaped plastic parts of particularly low stress. Compression processes are used in particular for the production of high-quality thin-walled and large-area shaped parts.